Do I really need matched pairs

[BigE]

What I did was:

For N channel:

9V+ to one end of 1K resistor, other end of 1K resistor to drain.

Gate and source tied to battery 9V-

DVM measurment across 1K resistor on mV scale = mA Idss.

For P Channel, reverse the battery.

 

[nr12]

Double post.

 

[Zen Mod]

Zen,

Fair statement. I have built two sets of ACA's 1.1 and 1.6 with perfect results, couple pre s and so forth. You're right. Boom makes me think, but I m going to try, there are great schematics and build guides as well. Slow and steady, will make two great amps. If not, I may not need my dfib machine anymore.. I appreciate the help and caution. I will report back..

what I meant is - read , ask , learn , build confidence and knowledge ...... much better than BigBadaBoom or abandonment

🙂

What I did was:

For N channel:

9V+ to one end of 1K resistor, other end of 1K resistor to drain.

Gate and source tied to battery 9V-

DVM measurment across 1K resistor on mV scale = mA Idss.

For P Channel, reverse the battery.

1K is too much - JFet is starved

1R or 10R max are sufficient for proper readout while not influencing result

 

[BigE]

Thank you Zen mod. Of course! There is a max current flow through 1k of just 9 ma....not enough for all types.

 

[Zen Mod]

that's not main problem

problem is that you're killing voltage for part , and it is voltage starved ..... also making measurement inconsistent ... every part is having different (effective ) drain voltage

 

[wdecho]

if you bought the bulk jfets, chances are you will not find a matched pair.
They are remnants of the matched jfets.

I have not found that to be the case. I have bought set of 8 ,k170and j74, from the store on at least 2 occasions and have been able to find a couple of close enough matched sets from each purchase. May not be the case every time but that is what I have found when I buy them from the store.

 

[jonparkhurst]

I appreciate everyones input. I have found the match pairs I need with in the larger sets and have some insight on news ones......
They are within a very small tolerance that is similar to what we are discussing. I soldered the legs and set up the 9 v battery as described above. Multimeter worked perfect and easy to desolder too.

JP

 

[vdi_nenna]

jonparkhurst, go to a electronics/surplus store or online and buy some test leads with alligator clips. If you want to build more stuff, you'll be glad you did.

Alligator Test Leads - Multicolored (10 Pack) - PRT-12978 - SparkFun Electronics

 

[Trismos]

I believe there's a misunderstanding about the difference between A and B grade Jfets, the assumption being A is better than B. Clarification on that would be helpful.

Perhaps someone might explain how these two work together in the F5 as well, and why tolerances should be within a certain amount. I do learn alot just trying to figure out things on my own - googling what Jfets and Mosfets are and all that good stuff. I learn alot more asking these type of questions here.

 

[Zen Mod]

same as GR, BL and V for Toshiba , for LS - A,B C are range groups by Idss

read datasheets, there is always something new to learn from

resistor values for JFet drains in F5 are optimized for BL range ...... having major impact on open loop gain (OLG) , thus on damping factor etc.

 

[GASCo]

Fair to assume you can always adjust resistors for different Idss?

 

[Zen Mod]

yup...... down to some not so wimpy Idss

using GR JFets , I would go to paralleling them (two instead of one BL)